rb_class_inherited_p returns Ruby's true/false/nil
rb_class_inherited_p returns Ruby's true/false/nil

From: https://github.com/ruby/ruby/blob/6b86549df8f6d48eeab3c7b48b3fd9ee02f744ba/object.c#L1828-L1846

> Returns <code>nil</code> if there's no relationship between the two.
ruby/spec,nobu/rubyspec,eregon/rubyspec,ruby/spec,nobu/rubyspec,nobu/rubyspec,eregon/rubyspec,ruby/spec,eregon/rubyspec